Amorphous oxide semiconductor thin films have high carrier mobilities, wide bandgaps, and can be formed at low temperatures compared with amorphous silicon, and are to be applied to a next-generation display to which large size, high resolution, and high-speed drive are required.
Among the oxide semiconductor thin films, an amorphous oxide semiconductor thin film including indium (In), gallium (Ga), zinc (Zn), and oxygen (O) (hereinafter, also referred to as In—Ga—Zn—O or IGZO) has been preferably used because of its extremely high carrier mobility. For example, in the disclosures of NPTLs 1 and 2, an oxide semiconductor thin film including In, Ga, and Zn (In Ga:Zn=1.1:1.1:0.9 in atomic percent) is used as an active semiconductor layer of TFT. PTL 1 further discloses an amorphous oxide containing Mo and an element such as In, Zn, Sn, or Ga, in which an atomic composition ratio of Mo to the total number of metal atoms in the amorphous oxide is 0.1 to 5 atomic percent. PTL 1 discloses a TFT having an active layer comprising IGZO and Mo.
Properties of the oxide semiconductors are, however, known to vary depending on various deviations in the course of film formation process and subsequent heat treatment. For example, TFT characteristics are liable to deviate by significant change of carrier concentrations, a dominant factor of TFT characteristics, caused by lattice defects and hydrogen in the film generated in the course of the film formation process. It is thus essential from the point of view to improving the productivity to evaluate properties of deposited oxide semiconductor thin films, to feedback the results of the evaluation, to adjust manufacturing conditions, and to control film quality in the manufacturing process of the display devices or the like.
In typical characterization methods, mobility and carrier density of oxide semiconductor thin films are evaluated by Hall-effect measurement after forming a gate insulating film or a passivation insulating film on an oxide semiconductor thin film, and an electrode having a predetermined size on the insulating film via lithography using a metal mask.
It takes, however, time and cost to form contact electrodes in such a contacting type evaluation method. Formation of the contact electrodes is also liable to induce additional defects in the oxide semiconductor thin film. It is thus required to establish a contactless-type evaluation method in which formation of contact electrodes is not necessary from the point of view to improving fabrication yield.
The existing evaluation methods involving the electrode provision suffer from difficulties such as low spatial resolution and long measurement time.
PTL 2 discloses a method for controlling film quality in a noncontact manner without forming an electrode, in which mobility of an oxide semiconductor thin film is qualitatively or quantitatively evaluated by a microwave photoconductive decay method.